Shine (US$9.99 in the Mac App Store) is not the first app we've seen that allows you to add realistic lighting effects to your images, but it is very capable and easy to use.
To start, drag and drop an image into the Shine palette, and place a light source. Drag it and rotate it where you want, then adjust size, intensity and color, and you'll have an enhanced image. The app includes undo, auto-saving and restoring.
The lighting effects are realistic and natural looking. The danger is that, like any app that modifies an image, you can get carried away. I prefer a subtle hand. My favorite place for photography is Antelope Canyon near Page, Ariz. It's a slot canyon, and you have to be there at just the right time and season to see the light rays come in from above. I always seem to be at the right place at the wrong time, so I was able to add the light rays to my photos after the fact. I liked the results.
I've previously reviewed LensFlare studio ($19.99) for the Mac, and also found it excellent. It's twice as much to buy as Shine, and has more effects, but it terms of quality and flexibility Shine is first rate.
Shine responds in nearly real time, and was very stable. I like the ability to show the edited and original photo on the same screen. My only complaint is that the app opens to a very small window on my 30-inch monitor, and even though you can enlarge it, the size isn't remembered as you go to work on a new image. That's an easy fix, and I hope it gets made.
Adding lens flares is something you won't do on every picture, but it can be very powerful when used appropriately. Shine does what it advertises, and does it well. I've added it to my small collection of permanent apps for post-processing images after they have been sharpened and color balanced in Photoshop.
Shine requires OS X 10.6.6 or later. It's well worth the asking price.